Nonlinear band gap transmission in optical waveguide arrays

The effect of nonlinear transmission in coupled optical waveguide arrays is theoretically investigated and a realistic experimental setup is suggested. The beam is injected in a single boundary waveguide, linear refractive index of which (n(0)) is larger than refractive indexes (n) of other identical waveguides in the array. Particularly, the effect holds if omega(n(0)-n)/c>2Q, where Q is a linear coupling constant between array waveguides, omega is a carrier wave frequency, and c is a light velocity. Numerical experiments show that the energy transfers from the boundary waveguide to the waveguide array above a certain threshold intensity of the injected beam. This effect is due to the creation and the propagation of gap solitons in full analogy with a similar phenomenon in sine-Gordon lattice [Phys. Rev. Lett. 89, 134102 (2002)]].     

利用耦合波导列提高光子晶体波导辐射

将耦合波导列应用于光子晶体单模波导,提出一种提高光辐射的光子晶体结构. 基于时域有限差分方法的理论研究表明,当将耦合波导列附加到单模光子晶体波导出口端的适当位置,使出射光分成若干强弱不一的光束,这些光束在传播空间通过干涉形成一定程度的汇聚,大大提高了光子晶体波导在水平方向的光辐射效率. 另外,当耦合波导列的行数大于某固定值(2N=8)时,辐射质量基本保持不变,由此可获得最紧凑的器件结构. 这种类型光子晶体在近场光学和集成光学等诸多方面有潜在的应用价值. 关键词： 光子晶体波导 光辐射 波导列 耦合波导     

Dual resonance mechanisms facilitating enhanced optical transmission in coaxial waveguide arrays

We experimentally and computationally demonstrate high transmission through arrays of coaxial apertures with different geometries and arrangements in silver films. By studying both periodic and random arrangements of apertures, we were able to isolate transmission enhancement phenomena owing to surface plasmon effects from those owing to the excitation of cylindrical surface plasmons within the apertures themselves.     

Penalty-free transmission in a silicon coupled resonator optical waveguide over the full C-band

We show that optical signal propagation in a coupled resonator optical waveguide (CROW) does not introduce additional performance penalties with respect to propagation in a conventional optical waveguide. The result is demonstrated in a low-loss (1 dB) eight-ring silicon CROW by means of bit error rate measurements performed over the whole telecommunication C-band. An extensive characterization of the CROW parameters in terms of insertion loss, bandwidth, in-band intensity ripple, crosstalk, and sensitivity to thermal drifts is presented.     

Gap solitons in waveguide arrays

Bright and dark spatial gap solitons are demonstrated in waveguide arrays. These gap solitons travel across the array at zero transverse velocity, in complete analogy with stationary (immobile) temporal gap solitons. Furthermore, the launching configuration for observing these stationary gap solitons is shown to be the analog of an "ideal experiment" for observing stationary temporal gap solitons, never observed so far. A clear distinction is established between the family of Floquet-Bloch solitons in general and discrete solitons in particular, and the limiting case of gap solitons.     

Topological modes in radiofrequency resonator arrays

Topological properties of solid states have sparked considerable recent interest due to their importance in the physics of lattices with a non-trivial basis and their potential in the design of novel materials. Here we describe an experimental and accompanying numerical toolbox to create and analyze topological states in radiofrequency resonator arrays including non-local coupling. These arrays are very easily constructed, offer a variety of geometric configurations, and their eigenfunctions and eigenvalues are amenable to detailed analysis. They offer well defined analogs to coupled oscillator systems in general in that they are characterized by resonances whose frequency spectra depend on the individual resonators, their interactions, and boundary conditions. A comparison of a small one-dimensional experimental system with theory by means of easy to measure S-parameters shows excellent agreement. The numerical toolbox allows for simulations of arbitrarily large systems, revealing an astonishing richness of band structures under systematic parameter variation.     

Critical coupling in plasmonic resonator arrays

We report critical coupling of electromagnetic waves to plasmonic cavity arrays fabricated on Moiré surfaces. Dark field plasmon microscopy imaging and polarization dependent spectroscopic reflection measurements reveal the critical coupling conditions of the cavities. The critical coupling conditions depend on the superperiod of the Moiré surface, which also defines the coupling between the cavities. Complete transfer of the incident power can be achieved for traveling wave plasmonic resonators, which have a relatively short superperiod. When the superperiod of the resonators increases, the coupled resonators become isolated standing wave resonators in which complete transfer of the incident power is not possible. Analytical and finite difference time domain calculations support the experimental observations.     

Discrete solitons in inhomogeneous waveguide arrays

The existence and dynamical properties of discrete solitons in inhomogeneous waveguide arrays with a Kerr nonlinearity are studied in two different configurations. First we investigate the effect of a longitudinal periodic modulation of the coupling strength on the dynamics of discrete solitons. It is shown that resonances of internal modes of the soliton with the longitudinal structure may lead to soliton oscillations and decay. Second we study the existence and stability of discrete solitons in arrays exhibiting a linear variation of the waveguide effective index in the transverse direction. We find that resonant coupling between conventional discrete solitons and linear Wannier-Stark states leads to the formation of so-called hybrid discrete solitons.     

Optical Bloch oscillations in waveguide arrays

We show that optical Bloch oscillations can emerge in waveguide arrays with linearly varying propagation constants. The existence of localized modes (Wannier-Stark states) with equidistant wave-number spacing (Wannier-Stark ladder) that do not undergo diffraction is analytically proved. The evolution of arbitrary initial excitations is described, and potential applications are suggested.     

Self-focusing and defocusing in waveguide arrays

We show that two regimes of diffraction exist in arrays of waveguides, depending upon the input conditions. At higher powers, normal diffraction leads to self-focusing and to the formation of bright solitons through the nonlinear Kerr effect. By slightly changing the input conditions, light experiences anomalous diffraction and is nonlinearly defocused. For the first time, self-focusing and self-defocusing have been achieved for the same medium, structure, and wavelength.     

Low-frequency sound field in a room with a Helmholtz resonator

The sound field generated by a point source of volume velocity in a room with a Helmholtz resonator is determined. The shift produced by the resonator in the natural frequencies of the room is calculated.     

Fano resonance in quadratic waveguide arrays

We study resonant light scattering in arrays of channel optical waveguides in which tunable quadratic non-linearity is introduced as nonlinear defects by periodic poling of single (or several) waveguides in the array. We describe novel features of wave scattering that can be observed in this structure and show that it is a good candidate for observation of Fano resonance in nonlinear optics.     

Higher-order solitons in amplitude-disordered waveguide arrays

We investigate the existence and stability of different families of spatial solitons in optical waveguide arrays whose amplitudes obey a disordered distribution. The competition between focusing nonlinearity and linearly disordered refractive index modulation results in the formation of spatial localized nonlinear states. Solitons originating from Anderson modes with few nodes are robust during propagation. While multi-peaked solitons with in-phase neighboring components are completely unstable, multipole-mode solitons whose neighboring components are out-of-phase can propagate stably in wide parameter regions provided that their power exceeds a critical value. Our findings, thus, provide the first example of stable higher-order nonlinear states in disordered systems.     

Discrete Talbot effect in waveguide arrays

We report the first observation of discrete Talbot revivals in one-dimensional waveguide arrays. Unlike continuous systems where the Talbot self-imaging effect always occurs irrespective of the pattern period, in discrete configurations this process is only possible for a specific set of periodicities. Recurrence of different input periodic patterns is observed in good agreement with theory.     

Raman-induced localization in Kerr waveguide arrays

We show that during the spatiotemporal compression in a periodic Kerr waveguide array, stimulated Raman scattering can effectively balance the effects of self-phase modulation, diffraction, and group-velocity dispersion, eliminating collapse and breakup over a wide range of input powers and leading to stable propagation in a single site.     

Image reconstruction in segmented waveguide arrays

A simple design of spatially finite one-dimensional and bidimensional waveguide arrays, showing self-imaging properties, is proposed. Image reconstruction insensitive to array truncation is achieved by the introduction of short waveguide interruptions (waveguide segmentation), which act as a focusing lens for discretized light.     

Using low intensity ultrasound to improve the efficiency of biological phosphorus removal

Low intensity ultrasound can produce various effects on biological materials, such as stimulating enzyme activity, cell growth, biosynthesis, etc., which may improve the efficiency of enhanced biological phosphorus removal (EBPR). We adopt total phosphorus (TP) and dehydrogenase activity (DHA) as indicators to confirm the feasibility of applying low intensity ultrasound in EBPR. Single-factor experiments and orthogonal test were conducted in batch anaerobic/oxic (A/O) process simulation to study the influence of ultrasonic intensity and exposure time in the EBPR process. The results showed that the optimal ultrasonic parameters were 0.2 W/cm² and 10 min under which condition the TP concentration in the effluent was 35–50% lower than that of the control (without ultrasonic irradiation). Changes of sludge activities after ultrasonic irradiation were examined. The improvement of sludge activity by ultrasound took 4 h after irradiation to reach the peak level, when an increase above 50% of DHA has been achieved by ultrasonic irradiation, and the enhancing effects induced by ultrasound disappeared in 16 h after irradiation. A tentative mechanism of biological phosphorus removal enhancement stimulated by ultrasound was discussed based on these phenomena.     

Using shock waves to improve the sound absorbing efficiency of closed-cell foams

Producing closed-cell foams is generally cheaper and simpler than open-cell foams. However, the acoustic and filtration efficiency of closed-cell foam materials is generally poor because it is very difficult for fluid or acoustic waves to penetrate into the material. A new method using shock waves to remove the membranes closing the cell pores (known as reticulation) and thus to improve the acoustic and filtration behavior of closed-cell foam material is presented. Various shock treatments have been carried out on polyurethane and polyimide foams and the following conclusions were drawn: (1) reticulation efficiency increased and thus the airflow resistivity and tortuosity decreased when increasing the amplitude of the shock treatment; (2) the rigidity of the foam is decreased; (3) the process is reliable and repeatable and (4) obtained acoustic performance is comparable to classical thermal reticulation.     

Discrete elliptic solitons in two-dimensional waveguide arrays

The fundamental properties of discrete elliptic solitons (DESs) in the two-dimensional waveguide arrays were studied. The DESs show nontrivial spatial structures in their parameters space due to the introduction of the new freedom of ellipticity, and their stability is closely linked to their propagation directions in the transverse plane.